A Zero-phase-sequence Current Transformer (ZCT) is generally provided in a power conditioner in order to detect ground fault current that flows when a ground fault occurs. When an abnormality occurs, however, such as the coil used in the ZCT becoming disconnected, a normal value ends up being output from the ZCT even when a ground fault occurs, and a ground fault cannot be detected. To address this issue, a known power conditioner includes an oscillation circuit that provides a pulse signal of a predetermined frequency to a series circuit formed by a ZCT and a resistor, a signal detection circuit that detects the pulse signal, a ground fault current detection circuit that detects ground fault current, and a self-diagnosis unit that detects whether the oscillation circuit and the ground fault current detection circuit technology has malfunctioned based on the signal detected by the signal detection circuit and the signal detected by the ground fault current detection circuit (for example, see Patent Literature 1).